Electrical Field of a Charged set of Spheres

[Snowman2526]

Homework Statement

Two spherical shells have a common center. A -1.37 x 10^-6 C charge is spread uniformly over the inner shell, which has a radius of 0.0422 m. A +5.86 x 10^-6 C charge is spread uniformly over the outer shell, which has a radius of 1.60 m. Find the magnitude of the electric field at a distance (measured from the common center) of (a) 0.200 m (b) 0.100 m, and (c) 0.025 m.

Homework Equations

Honestly I'm not sure how to set this equation up.

I have calculated the Electric Field of each sphere. the smaller is 6.92*10⁶ and the larger is 2.06*10⁴

I also calculated the Electric Flux each. The smaller being 6.54*10⁴ and the larger being 6.63*10⁵.

Now I'm not quite sure if I have to add the two spheres together, or what to do since the inside sphere is negatively charged.

 

[Delphi51]

This is a Gauss' Law question.
For (a) you imagine a sphere at with radius 0.2 and consider only the charge contained in it. The whole thing is spherically symmetric, so the E field will be the same everywhere on the surface of the sphere.

 

[Snowman2526]

i'm sorry to necro bump my old thread but I still cannot get this problem. When you say imagine it with the radius .2...do i add it to the radius of the two circles? or is it a fresh radius and I just plug .2 into r for my equation.

On top of that, I am sitting with two variables because I can't find out what E is. Do I add the Coulomb charge of the two spheres or combine them somehow? I'm honestly not looking for someone to do it...I just can't grasp the concept for this problem.

 

[Delphi51]

Imagine a sphere with radius 0.2. The Law says E times the surface area of the sphere equals some constant times the charge INSIDE the sphere. So you just ignore the outer charge for part (a).